The present invention relates to a process for producing high purity nickel hydroxide precipitate (NiHP) from nickel-containing feed solutions derived from the dissolution of nickeliferous lateritic ores or other nickel-containing feed materials such as concentrates or mattes, the NiHP being suitable for pelletization with iron-containing ores and for stainless steel production.
The addition of such suitably produced NiHP into iron containing ores (iron ore, laterite ore and/or mixtures thereof) allows for the production of nickel pig iron with high enough nickel content (8-20%) to make it suitable for the manufacture of 300 series stainless steels. For this to be successful, however, the nickel hydroxide feed must be available from an economically viable source and must be substantially free from impurities that are deleterious to stainless steel manufacture and performance.
Suitably produced high purity NiHP can be further processed to produce high purity UTILITY® nickel product. (UTILITY is a trademark of Vale Inco Limited).
There are a variety of known processes for metal extraction from nickeliferous lateritic ores. Examples include high pressure acid leaching (HPAL), atmospheric acid leaching (AL) or heap leaching (HL) of lateritic ores. These processes for nickel recovery from lateritic ores, produce a leach solution which contains, along with the nickel, a number of other impurities such as iron, aluminum, chromium, silica, copper, zinc and, most notably, cobalt and manganese.
A number of known separation processes are available for removing these impurities from the nickel-containing leach solution. These include metal hydrolysis and precipitation (which may include prior oxidation or reduction of dissolved metal species), sulfide precipitation, ion-exchange, or solvent extraction (SX).
Because nickel laterites are not amenable to significant upgrading (unlike most nickel-bearing sulfide ores), almost the entire ore must be leached, thus producing dilute, typically 3-7 g/L Ni, nickel-containing leach solution. It is therefore a major challenge to invent a simple and cost-effective process to recover the value metals from laterites.
The known options generally fall into two categories.
One category is the indirect route that produces an intermediate precipitate which contains nickel in a substantially more concentrated form after some of the metal impurities, typically Fe, Al and Cr, have been removed. The intermediate precipitate is then releached and the concentrated nickel-containing solution is further refined through several steps to produce purified nickel solution from which nickel is recovered to a marketable nickel product.
The other category is the direct route which produces the purified nickel solution without pre-concentrating via an intermediate precipitate.
One example of the direct route is used in the Bulong HPAL plant in Western Australia which uses solvent extraction to remove cobalt, manganese, copper and zinc from the dilute leach solution after Fe, Cr and Al removal by hydrolysis and precipitation, to produce a purified nickel solution from which nickel is transferred, by another solvent extraction process, into a concentrated solution suitable for nickel recovery by electrowinning.
The Vale Inco Goro Nickel HPAL process also uses the direct route approach. Impurities such as Fe, Cr, Al, Si and Cu are removed by hydrolysis and precipitation, and by ion-exchange for the remaining Cu, followed by solvent extraction to separate Ni, Co and Zn away from Mn and other impurities. This solvent extraction step also serves as a concentration step as it transfers Ni, Co and Zn into a concentrated chloride solution. This solution is then treated for Zn removal by ion-exchange, and finally Co is separated from Ni by another solvent extraction step to produce a pure nickel chloride solution from which Ni is recovered by pyrohydrolysis as a high-purity nickel oxide suitable for the stainless steel market.
In the indirect approach, nickel is precipitated as a mixed sulfide precipitate (MSP) or as a mixed hydroxide precipitate (MHP). Examples of processes based on MSP include one used in the Moa Bay plant in Cuba, the Murrin-Murrin plant in Western Australia and the Coral Bay Nickel plant in the Philippines. The MSP intermediate is further refined through a number of operating steps either within the same plant (e.g., the Murrin-Murrin plant) or it is shipped for refining elsewhere (as in the Moa Bay and Coral Bay plants).
For example, at the Murrin-Murrin HPAL plant, after iron removal, hydrogen sulfide is used to produce an intermediate mixed Ni/Co sulfide (also containing other impurities such as Cu and Zn). The mixed sulfide is then pressure leached under oxidizing conditions to produce a concentrated Ni solution, from which the impurities (Cu, Zn, Co) are sequentially removed by hydrolysis and solvent extraction to produce a purified nickel solution from which nickel is recovered by hydrogen reduction.
Both Cawse and Ravensthorpe HPAL plants in Western Australia use precipitation of nickel as intermediate nickel/cobalt mixed hydroxide precipitate or MHP, using MgO as a base, to separate the pay metals from manganese. The MHP is then redissolved in ammonia/ammonium carbonate solution and the solution is further treated through another series of refining steps, including hydrolysis/precipitation, solvent extraction, ion-exchange, etc., to produce purified nickel solution from which nickel is recovered to a marketable product (nickel oxide or nickel metal).
One advantage of the indirect route is that it provides a break in the process allowing for greater flexibility and operational independence of the front-end leaching and back-end refining circuits.
A disadvantage of the MSP based processes, however, is the generally high capital costs associated with building the necessary H2S plant and associated services. A further disadvantage is that subsequent hydrometallurgical refining of the MSP typically requires a costly pressure oxidative leaching step.
Producing an MHP intermediate does not suffer from these MSP disadvantages. However, the known commercially practiced MHP based processes (e.g., Cawse and Ravensthorpe) rely on ammonia chemistry to achieve additional separation of Ni and Co from Mn upon dissolution of MHP in the ammonia/ammonium carbonate solution because of the significant transfer of Mn into the MHP intermediate. For example, as reported for the Ravensthorpe project (see D. T. White, et al., “Impurity Disposition and Control in the Ravensthorpe Acid Leaching Process,” CIM Iron Control Technologies Symposium, Montreal, 2006, eds J. Dutrizac and P. Riveros), the Ni:Mn mass ratio increases only about three times across the MHP circuit, between the ratio in the solution feed to the MHP circuit (about 4.6) and the ratio in the MHP solids (about 14.4).
One disadvantage of the ammonia/ammonium carbonate releach of the MHP intermediate is the phenomenon of MHP ageing where there is notable reduction in the extent of Ni and Co redissolution from MHP that has been stored for a period of time.
A further disadvantage of the ammonia based chemistry is the technical difficulty, and associated significant cost, to meet increasingly strict environmental limits for ammonia in discharge effluent. This is particularly relevant for locations where operating a zero-discharge facility is not feasible.
Yet another disadvantage of the ammonia based chemistry is the need for ammonia recycle within the refining process. Ammonia recycle typically employs an ammonia lime boil operation and this operation is very energy intensive.
Accordingly, it would be beneficial to provide a simpler, and thus potentially more cost effective process which will produce an intermediate MHP product and then refine it to a purified nickel hydroxide precipitate (NiHP) product that is suitable for pelletization with iron-containing ores and also has sufficient purity such that it is directly suitable for stainless steel production, while avoiding the disadvantages and limitations of the ammonia-based MHP refining processes.
It would be a further advantage to provide a process whereby the purified NiHP can be produced from other nickel-containing feed sources such as concentrates or mattes.
It would be a further advantage to provide a process that is effective at producing and refining MHP derived from leach solutions of laterite ores of widely different compositions and therefore containing a wide range of impurities such as Cu, Zn, Fe, and Mn as well as cobalt.